Problem with Adsense ads showing up

I’m having problems with putting adsense ads on my blog. I’ve tried a number of plugins, but they all do the same thing. Then I tried just putting the code into a text widget, and it does the same thing. So, I believe it’s something other than plugins. The ads show fine in posts and on the second page of posts, but not on the index page. And I can’t for the life of me figure out what’s wrong. Does anyone have any clue as to why it isn’t working right?

http://rambling-theory.com is my site.